1. Field of the Invention
The present invention relates to an outer-loop control device in a CDMA mobile communications system and a method thereof.
2. Description of the Related Art
Recently, a CDMA mobile communications system has been spotlighted as a next-generation communications system.
In a CDMA mobile communications system, a plurality of channels share one frequency band. In this case, each channel is identified by a spreading code attached to it. Therefore, an interference noise power varies depending on the number of simultaneous user.
Generally, the longer a propagation distance is, the larger the power attenuation of a radio wave is. The instantaneous power value of a receiving signal also changes due to multi-path fading and the like. Therefore, it is difficult to stably maintain the communications quality of a mobile station connected to a base station at a desired level.
In order to follow such a large change in the number of interfering users and instantaneous value changes due to multi-path fading, a closed loop transmission power control (TPC), for controlling an SIR in such a way that an SIR (signal to interference ratio) on a receiving side may approach a reference SIR, is exercised by measuring a signal-to-interference power ratio (SIR) on the receiving side and comparing the measurement value with the reference SIR.
However, an SIR needed to obtain a desired quality (block error rate) varies with the change of travel speed in communications or the change of a propagation environment during travel. A block error is observed in order to compensate for this change. If the observed value is higher than a desired BLER (target block error rate), the reference SIR is increased. If the observation value is lower than a desired BLER, the reference SIR is decreased. Flexibly controlling a reference SIR in this way is called outer-loop control.
For the outer-loop control method described above, the following methods have been used.                (1) Technical Report of IEICE, RCS98-18, pp. 51-57        (2) The 1999 General Conference of The Institute of Electronics, Information and Communication Engineers, B-5-145        (3) The 2000 General Conference of The Institute of Electronics, Information and Communication Engineers, B-5-72, etc.        
In the description below, the following symbols are used.
T: Block error observation time period
BLER: Target block error rate
Sinc: Unit increment in the case where a reference SIR is increased at the time of a reference SIR update
Sdec: Unit decrement in the case where a reference SIR is decreased at the time of a reference SIR update
In method (1), a reference SIR is increased/decreased depending on the number of block errors occurring in a predetermined observation time period.
In method (2), it is detected whether there is an error in each block. If there is an error, the reference SIR is increased. If there is no error, the reference SIR is decreased.
In method (3), if there is an error in an observation time period T calculated by T (block error observation time period)=round (ln 2/BLER), the reference SIR is increased. If there is no error, the reference SIR is decreased. The word “round” means to count fractions of 5 and over as a unit and to disregard the rest.
Table 1 shows the comparison result of these methods.
TABLE 1Features of Prior Arts[1][2][3]observationconstant1constanttime periodT = round (lnof block2/BLER)error rateReferenceWhen anFor each blocktime ofSIR updateobservationerrortimingtime periodoccurrencecomes to anwhenendobservationtime periodendsUpdate stepVariesIncrement ≠increment =dependingdecrementdecrementon theBLER ×number ofSinc =errors(1 − BLER) ×occurringSdecin anobservationtimeperiod.
In method (1), a reference SIR update interval is fixed and is long. Both the update interval and the increment/decrement of the reference SIR are empirically determined. No theoretical ground is shown.
In method (2), the size of Sdec is sometimes too small compared with Sinc. For example, if a target block error rate, the increment Sinc of a reference SIR are 0.01 and 1 (dB), respectively, the decrement Sdec of the reference SIR becomes approximately 0.01 (dB) and is too small. If the value must be implemented by hardware and if both SIR measurement accuracy and a reference SIR control step are taken into consideration, it is not practical to finely control the reference SIR in this way.
In method (3), if a BLER is given, an observation time period T is uniquely determined. If there is an error, the reference SIR can be immediately updated. However, if there is no error, the set observation time period cannot be updated. Therefore, if a BLER is low, au update interval becomes fairly long.